Answer: Option(d) is correct option
Explanation:
According to the question,Cora should give the complete speech first so that audience can listen to it and attain information about marbles.After she finishes the speech she can let audience come on stage to have a look on her marbles in coordinated manner if no other speaker has to give speech further.This will not create chaos among audience and disturbance will be eliminated.
Other options are incorrect because passing marbles among audience ,letting volunteers carry marble to audience and gathering audience to see her marbles in group during her speech will tamper her focus from speech and audience might lose interest from speech and notice marbles only.Thus, the correct option is option(d).